To gain a deeper understanding of the mechanisms underlying associations between allozyme genotypes and rates of respiration in Lolium perenne, Vmax Km and rates of glucose flux through glycolysis and the pentose phosphate pathway were estimated for the three genotypes of the 6pgd locus. Km Vmax and Vmax/Km differed significantly among genotypes. Values of Km for the 11, 12, and 22 genotypes were 0.29, 0.25, 0.13, while the values of Vmax/Km for die 11, 12, and 22 genotypes were 3.79, 3.85, 6.70. Flux through the pentose shunt did not differ among genotypes at 20 °C, but at 35 °C the rates of flux for the 11, 12, and 22 genotypes were 0.15, 0.25 and 0.42, respectively. Thus, the 6PGD allozyme genotypes differ markedly in both enzyme kinetic characteristics and in flux through a metabolic pathway. These associations reveal potentially causal relationships between allozyme genotypes and rates of respiration.